Field of the Invention
The present invention relates to solid thermoset foams obtained by chemical reaction and foaming of an expandable composition containing a polyol component, a polyacid component, a surfactant and an esterification catalyst.
Description of the related art including information disclosed under 37 CFR 1.97 and 1.98
It is known to prepare thermoset polyester foams by reaction of a polyol and of a polyacid.
In application WO 2010/059925, a highly crosslinked polyester is obtained in two steps: the first step consists in forming a non-crosslinked prepolymer by reaction of a polyol and of a polyacid at a first relatively low temperature and the second step in crosslinking the prepolymer either at a second temperature higher than the first, or at a moderate temperature in the presence of a crosslinking agent or a polymerization (esterification) catalyst. The absence of the catalyst in the first step appears to be essential for being able to carry, out this process in two steps.
Certain cured polymers are described as foams. Their density is however relatively high (Example 12: 350 kg/m3; Example 16: 190 kg/m3). The use of physical or chemical foaming agents is envisaged (see [0040]).
International application WO 2012/052385 describes an expanded polymer obtained by reaction of glycerol and citric acid at a temperature between 80 and 130° C. until the degree of conversion is at least equal to 90%, optionally in the presence of an esterification catalyst. When a degree of conversion of at least 90%, preferably 98%, is achieved, the reaction mixture is heated at a higher temperature, close to 150° C. The first step at low temperature is intended to avoid, insofar as possible, the decarboxylation of the citric acid.
The process described in this document requires very long reaction times, ranging from several hours to several tens of hours. The polymerization is preferably carried out in non-stick molds, for example made of Teflon. The foams thus obtained have a density of between 200 and 850 g/liter.
International application WO 2013/121033 describes an improvement of this process where the polymerization of glycerol and citric acid is carried out in contact with a substrate with an outer layer comprising at least a metal, a metal oxide or a metal halide. This process provides foams having a more regular pore size than those prepared by the process described in WO 2012/052385. The densities claimed are between 50 and 850 g/liter. The foams prepared in the examples according to the invention have densities between 282 g/liter and 482 g/liter.